You don't need to do Enterprise Activation at all. In fact, just hide that icon. It will cause problems for you if you try to use it without a BlackBerry Data Plan, or an enterprise server to attach to.

You don't need an unlock code, because the device is a T-Mobile device, and you have a T-Mobile SIM card.

If you can't call out using the phone, then maybe there's some security restrictions on the phone from the previous owner.

Go into Options > Security Options > General Settings and tell us if it says anything next to IT Policy Name: